Not sure the specific size as I don't have one yet, but as it comes in 2 halves, and each one folds once, they will not fold small enough to fit in the starter box.
Are you sure? I am fairly certain that Colby had said in another thread (might be here on BGG, or on the Plaid Hat forums) that the board comes in 2 halves BUT each half of the board then effectively folds half then half again. If you get what I mean.
Stacked you then have 8 "layers", which in the prototype he was indicating was about an inch and a bit thick in height.
I am thinking that if the play field dimensions are much the same, then provided the board edges are mot much bigger than the paper board, then it might just fit in the box. BUT the box might not be deep enough.
Still as you say we don't have one yet.